Add mutations after adding new individuals to a tree sequence

[szhan]

The function add_individuals_to_tree_sequence only adds new edges to an existing tree sequence but not new mutations. A separate function needs to be implemented to add new mutations that occur above newly added sample nodes of the individuals.

[szhan]

It should be as simple as getting the inherited state of the parent node and checking whether it is identical to the observed state of the sample at each site.